An "S" Model convotherm means it is a "spritz" or "spray" model. There is NOT a boiler to pour descaler into. Thats why there is not a "descale" port on top. What you are seeing is exactly as it came from the factory. A model "OGB" would have a boiler and a descale port on top. And being a spritzer, there are no water probes. Very likely an incoming water problem. Filters or inlet screen.
Otherwise, remember thats 20 psi DYNAMIC.
Though the nozzles inside do tend to calcium up easily, that would not cause a low pressure issue.
Hope you made out ok on it.